FIELD OF INVENTION:
This invention relates to a method of manufacturing a gear.
In a conventional manufacturing method for automotive components, such as engine components including gears, accuracy of dimensions and their relationship with each other is required to ensure control of output component quality. Closely toleranced components, such as gears, connecting rods and crank cases, require precision machining to measure dimensional accuracy and reliability in use.
Conventionally, the bore generation of a gear forms an important part of manufacturing any gear. High precision is necessary. In particular, a precise relationship must be maintained between critical gear dimensions such as pitch circle diameter (PCD), and the bore diameter. It is conventional manufacturing practice to generate the bore in relation to the PCD to ensure concentricity and associated quality aspects. Proper functioning of a gear requires a close concentricity between its bore and its PCD. Any eccentricity leads to gear noise and higher wear and tear, both undesirable outcomes.
Normally, the sequence of manufacturing a gear involves generation of the OD portion of the gear in a pre-machining stage along with the initial bore of the gear. This is followed by cutting of the gear teeth, with the PCD portion being generated at the hobbing stage. The bore of the gear is then finished through a machining operation following heat treatment.
Heat treatment plays an important role in manufacturing a gear, as it imparts necessary metallurgical properties. During such heat treatment, distortions in various gear dimensions, including OD and PCD, occur at different magnitudes owing to non-uniform distribution of material particles and their physical properties. Therefore, it is difficult to achieve the required concentricity, not assisted by using the distorted PCD portion of the gear as a

clamping reference. Moreover, loading the gear on a PCD chuck is cumbersome for the operator and automation of the process is difficult. In particular, the pins used to locate the PCD of the gear are subject to wear and tear owing to repeat use resulting in inaccurate and time consuming setting of the centre of the chuck to obtain acceptable concentricity.
OBJECT OF INVENTION:
It is an object of the present invention to provide a method for manufacturing a gear that is less prone to the problems identified above.
STATEMENT OF INVENTION:
With this object in view, the present invention provides a method of manufacturing a gear having an inner bore, gear teeth with a common pitch circle diameter and a gear portion of outer diameter comprising:
supporting the gear by reference to the outer diameter, and machining a bore whilst said gear is so supported. In this manner, gears of required concentricity may be produced and difficulties for the operator, encountered in using the pitch circle diameter for support or location, avoided.
Preferably, the common outer diameter encircles the tips of the gear teeth.
Preferably, the gear undergoes a heat treatment process before its bore is finish machined. As the outer diameter has been used for support or location of the gear, by clamping, there is less influence of distortion during heat treatment on precision and concentricity. Specifically, the Applicant has found that the distortion of the OD portion of the gear after heat treatment is more uniform than the PCD. While the distortion of the OD portion was always found to be within 15 microns, the distortion of PCD was about 35 microns. Accordingly, a gear of superior quality may be manufactured.

The outer diameter and pitch circle diameter may be generated simultaneously. Such generation may be achieved using a common cutter (or topping hob), hobbing being the preferred machining operation. Cutting of the teeth is also conducted during the hobbing operation.
A gear produced in accordance with the above method also forms part of the present invention. The gear may be applied in various ways and is of particular use in automotive applications.
DESCRIPTION OF INVENTION WITH REFERENCE TO DRAWINGS:
The gear manufacturing method of the present invention may be more fully understood from the following description of preferred embodiments made with reference to the accompanying description in which:
Figure 1 illustrates a support or clamping arrangement used during bore finishing of a gear manufactured in accordance with the prior art;
Figure 2 illustrates a support or clamping arrangement used during bore finishing of a gear manufactured in accordance with one embodiment of the present invention;
Figure 3 illustrates dimensional accuracy, and obtained concentricities, obtained in accordance with a prior art gear manufacturing operation involving bore finishing with the clamping arrangement shown in Figure 1;
Figure 4 illustrates dimensional accuracy, and obtained concentricities, obtained in accordance with a gear manufacturing operation involving bore finishing with the clamping arrangement shown in Figure 2;

Figure 5 illustrates typical geometry of a prior art hob used for manufacture of a gear; and
Figure 6 illustrates hob geometry for a hob used on manufacturing a gear in accordance with one embodiment of the present invention.
Referring now to Figures 1 and 2, a conventional gear manufacturing operation is described with reference to its attendant difficulties. Gear 1 is fabricated with a pitch circle diameter ("PCD") 2 and a semi-finished bore 3. After heat treatment, the concentricity between the bore 3 and the PCD 2 is within 0.070 mm. The gear manufacturing operator then clamps gear 1 onto PCD clamping chuck 4 using pins 5. Pins 5 are located and held in pin holders 6, helping in supporting and locating the PCD 2 of gear 1, for bore finishing. This clamping operation is cumbersome and achieved with difficulty. If performed on many occasions, pins 5 wear making the operation even more difficult and causing less precision of the final gear 1, particularly with reference to the required concentricity. Further the distortions after heat treatment, being more at PCD than at OD result in increased inaccuracy of finishing the bore in a prior art manufacturing method, which employs PCD as the reference point for finishing the bore. On mounting of the clamping arrangement on a turning machine, bore 3 of the gear 1 is finished.
The dimensional accuracy achieved by this method, at the generation stage, is about 0.15mm and the concentricity between the bore 3 and outer diameter is about 0.2 mm for a typical gear as illustrated in Figure 2.
Referring to Figures 3 and 4, there is described a gear manufacturing method in accordance with the present invention. In this method, gear 10 is supported by reference to its outer diameter 18, and bore 19 of the gear is machined whilst gear 10 is so supported. During the machining operation, the OD 18 of gear 10 is generated simultaneously with PCD 12 while teeth are cut by hobbing, using a topping hob (not shown).

Gear 10 is clamped on an OD clamping chuck 14, the jaws 16 of which clamp the gear 10 at the OD 18. After heat treatment, the concentricity between bore 19 and PCD 12 is within 0.070 mm and the concentricity between bore 19 and OD 18 is within 0.045 mm. This clamping operation is relatively easy for the gear manufacturing operator to achieve and the use of pins is avoided. Jaws 16 are used instead making the clamping arrangement less cumbersome. This clamping arrangement is then mounted on a turning machine to finish size of the bore 19 of gear 10. In this manner, the finish bore 19 is generated with reference to the OD 18 of the gear 10.
Dimensional accuracy achieved for a typical gear 10 manufactured by this method is about 0.05 mm and the concentricity between the bore 19 and OD 18 is about 0.036 mm as illustrated in Figure 4. The dimensional accuracies and concentricities are therefore superior to those achievable using the prior art method.
The salient difference between the tooth profile of the hob in the prior art condition and the new system is that the tooth depth of the hob is higher in the hob which was being employed in the prior art system thereby the OD of the gear does not get altered or does not get generated during the hobbing process. Whereas in the new system, the hob tooth depth is exactly same as the tooth depth of the gear to be hobbed thereby generating the OD of the gear while tooth generation during hobbing operation. Figure 5 illustrates the tool geometry of the hobbing cutter according to prior art and Figure 6 illustrates the tool geometry of the hobbing cutter according to the invention. It can be observed that the tool depth of hobbing cutter in Figure 5 is more as compared to the tool depth of hobbing cutter used according to invention for a given gear OD, that is, the tool depth (5.20 shown in Figure 5) according to prior art is more than that of gear tooth depth to be generated on the gear,

while according to invention, the hobbing cutter has a depth (4.720 as shown in Figure 6) equal to gear tooth depth to be generated on the gear. Thus, in a prior art system, the OD of the gear is not generated simultaneously with the tooth of the gear. However, the method employed in the invention has the OD of the gear generated simultaneously with the generation of the tooth profile. As the OD is generated while generating the gear teeth, the outer surface of the OD is used for precisely locating the gear while machining/finishing the bore dimension of the gear, thereby producing a gear with better precision and having requisite relationship between different dimensions such as concentricity between OD and bore. This eliminates problems faced while manufacturing the gear according to prior art as mentioned above.
Modifications and variations to the measurement method and apparatus of the present invention may be apparent to the skilled reader of this disclosure. Such modifications and variations are within the scope of the present invention.